This invention relates generally to nuclear reactor fuel assemblies and more particularly to providing a wear sleeve in the control rod guide tube of an assembly whereby vibration of the control rod will not damage the tube.
Nuclear reactors of the pressurized-water type typically have a core region consisting of a multiplicity of vertically oriented fuel assemblies, each assembly containing a matrix of fuel elements. The assembly skeleton includes a plurality of elongated guide tubes to which are connected grids for supporting the fuel elements, and end fittings for securing the assembly between vertically spaced support plates. Each guide tube also serves as a sheath for a control rod which is inserted into the core for the purpose of controlling the heat output of the fuel. An upward flow of liquid is maintained in the guide tube to cool the control rod. Examination of selected fuel assemblies during the refueling of some reactors of this type has revealed the existence of wear patterns on the inside of the guide tubes at the elevation corresponding to the position of the control rod tip within the tube when the rod is in the upper limit of travel, i.e., the unique "withdrawn" position. Such wear behavior has the potential for weakening or perforating the guide tubes. Significant weakening of the tubes is of particular concern during refueling when the full weight of the assembly is borne by its guide tubes.
It is believed the most straight-forward way of avoiding excessive wear of the guide tube is to provide a wear sleeve in the vicinity of the critical region of the tube. It is not a simple matter, however, to design a wear sleeve that is compatable with the many functions a guide tube must perform in a hostile environment. The present invention overcomes these design problems.